Humanity has also had a great time making different kinds of plastics, only to discover that they are also penetrating our body at a microscopic scale.
While we don't have the smoking gun yet (i.e. radium jaw), it's at least scary to me, because platics are so pervasive across the population that if a smoking gun DOES emerge, it might be too late. I think the article itself brings up another good example: asbestos.
I get all the jokes about dihydrogen monoxide killing people who breathe it, i.e. drowning, but this is fundamentally different, because we are discovering a thing that is interacting with our bodies in a way that was not historically present throughout evolution. Presumably if we'd evolved in the mist of nano plastics there wouldn't be much cause for worry, because a priori we'd have survived and thrived in its presence.
Even if nano plastics did exist for the last billion years, it wouldn't mean they're safe, just that they don't kill people faster than they can reproduce.
It's amazing how accepting people are of dangerous substances...